1. Field of the Invention
This invention relates to a sputtering system having a plasma emission monitor and a process for producing deposited films by using the system.
2. Related Background Art
In the production of transparent conductive films, a vacuum vapor deposition process making use of electron beam heating and a sputtering process of, e.g., a DC magnetron type are in wide use. Vapor deposition, however, is commonly unsuitable for mass production because the transparent conductive films are formed at a low deposition rate. On the other hand, DC magnetron sputtering can form the films at a higher deposition rate. In this process, an electric power is applied between a target and a substrate or a vacuum chamber to cause plasma of sputtering gas to sputter the target.
ITO (indium-tin oxide) films, an example of the transparent conductive films, are widely used as transparent electrodes for, e.g., liquid-crystal display devices, electroluminescent (EL) display devices, electrochromic display devices, photosensors and solar cells. When such ITO films are deposited by DC magnetron sputtering, a reactive sputtering process using a metallic indium 90: tin 10 target or sputtering process using an ITO ceramic target is available. It is reported that the former process can enjoy a lower target cost than the latter process and is suitable for mass production (J. Czukor, W. Kittler, P. Maschwitz and I. Ritchie, xe2x80x9cThe Effects of Process Conditions on the Quality and Deposition Rate of Sputtered ITO Coatingsxe2x80x9d, Proc. Annu. Tech. Conf. Soc. Vac. Coaters, Vol. 34th, pp.190-195, 1991).
In the reactive sputtering, it is known to use a Plasma Emission Monitor (PEM) to control the flow rate of reactive gas (S. Schiller, xe2x80x9cProgress in the Application of the Plasma Emission Monitor in Web Coatingxe2x80x9d, 2nd Int. Conf. on Vacuum Web Coating, 1988). The PEM is a device for monitoring electric signals of plasma emission whose light is collected by a collimator and photoelectrically converted by a photomultiplier tube (PM) (or multiplier phototube). The PEM is so designed as to be set at a certain sensitivity to monitor the emission intensity of plasma. Using the PEM, the feed and flow rate of reactive gas, i.e., oxygen gas, are so very delicately regulated that the plasma emission intensity can be kept constant, whereby a homogeneous film can be formed.
Meanwhile, a DC magnetron sputtering system using a cylindrical rotary target is also known in the art. Use of this system is known to enable achievement of a higher deposition rate and a higher target utilization efficiency (U.S. Pat. Nos. 4,356,073 and 4,422,916).
Hitherto, when ITO films are deposited on substrates by magnetron sputtering, conditions such as electric power to be applied, flow rate of sputtering gas and sensitivity of PEM are kept fixed even when substrates are changed. Then, it has been difficult to keep the film thickness of each deposited film constant on each substrate. Non-uniformity in film thickness, especially of ITO films deposited on the surfaces of photovoltaic devices, may bring about interference color of light to cause a poor appearance. This non-uniformity appears especially when films are formed over a long time as in the case where thick films are formed, or where films are formed on continuous-sheet substrates, using a rotary target in both cases.
Accordingly, an object of the present invention is to provide a sputtering process that can be free from any difference in film thickness and layer quality even after films are formed over a long time.
To achieve the above object, the present invention provides a process for producing a deposited film, comprising the steps of:
in the state where a substrate is set in a film-forming chamber, introducing a sputtering gas containing no reactive gas into the film-forming chamber and causing discharge therein;
adjusting the sensitivity of a device for monitoring emission intensity of plasma of the discharge, in such a way that the device reads a set value; and
introducing at least a reactive gas into the film-forming chamber to deposit a film on the substrate by subjecting a target to sputtering while controlling the feed rate of the reactive gas in such way as to provide a constant deposition rate.
The present invention also provides a process for producing a photovoltaic device having at least a substrate, a semiconductor layer and a transparent electrode; the transparent electrode being formed by a process comprising the steps of:
in the state where a substrate having a semiconductor layer is set in a film-forming chamber, introducing a sputtering gas containing no reactive gas into the film-forming chamber and causing discharge therein;
adjusting the sensitivity of a device for monitoring emission intensity of plasma of the discharge, in such a way that the device reads a set value; and
introducing at least a reactive gas into the film-forming chamber to form a transparent electrode on the substrate having a semiconductor layer by subjecting a target to sputtering while controlling the feed rate of the reactive gas in such way as to provide a constant deposition rate.
The present invention still also provides a deposited film producing system comprising a film-forming chamber, means for holding a substrate in the film-forming chamber, means for monitoring emission intensity of discharged plasma the sensitivity of which is adjustable, a gas feed means, means for measuring deposition rate, means for controlling the feed rate of a reactive gas in such way as to provide a constant deposition rate, and means for subjecting a target to sputtering to deposit a film on the substrate.